This invention relates to the electrodeposition of tin, lead, and tin-lead alloys, and particularly to a plating bath which is free of fluoborates and which deposits smooth, level and bright tin and/or lead coatings.
Aqueous acidic plating baths for depositing tin and/or lead coatings on substrates have been known in the art, and most of these baths contain, in addition to the water-soluble tin and/or lead salt, fluoborates, fluosilicates, sulfates, sulfamates, etc. Plating baths containing fluoborate have been used widely to permit high-speed, uniform metal plating of tin, lead or tin-lead alloys. However, baths containing fluoborates generally are quite corrosive and toxic requiring special equipment which is expensive and presents difficulties in operation, including the disposal of waste water.
In addition to the basic ingredients, the prior art has suggested improvements in tin and/or lead plating baths by including additives which will improve on the brightness of the deposit obtained from such baths. For example, in U.S. Pat. No. 3,875,029, the use of naphthalene monocarboxaldehyde either alone or in combination with certain substituted olefins described in the patent results in an improvement in the brightness of the deposit. Other ingredients which have been suggested as being useful additives in tin and/or lead plating baths include various combinations of aldehydes, ketones, nonionic surfactants, and amines. The plating bath described in U.S. Pat. No. 3,785,939 for producing bright deposits of tin-lead alloys comprises a combination of a nonionic polyoxyalkylated surfactant, a lower aliphatic aldehyde, an aromatic aldehyde, and an amine.
More recently, plating bath formulations have been suggested which provide for the deposition of tin and/or lead coatings wherein the baths contain no fluoborates. For example, U.S. Pat. No. 4,459,185 describes a plating bath which is prepared by adding one or more specified cationic, amphoteric and/or nonionic surfactants and one or more leveling agents to a principal plating bath which contains an alkane sulfonic or alkanol sulfonic acid and either a divalent tin salt or a divalent lead salt of the sulfonic acids or a mixture of these. The patentees suggest that the performance of such plating baths is comparable or even superior to plating baths containing borofluoride. The plating baths contain certain specified surfactants which may be cationic surfactants, amphoteric surfactants, or nonionic surfactants. The smoothness of the coating is improved by the synergistic effect obtained by the combination of the specified surfactants and certain leveling agents. A variety of leveling agents including benzaldehyde derivatives is described in the patent.
Tin-lead alloy plating baths including hydroxyalkyl sulfonic acids are described in U.S. Pat. No. 4,132,610. The baths do not contain fluoborates or phenol sulfonates, but do contain the reaction product of o-toluidine with acetaldehyde as a brightener.
U.S. Pat. No. 3,905,878 describes tin-lead plating baths containing at least one sulfonate salt of tin and lead as well as free phenolsulfonic acid and/or free cresolsulfonic acid. Additionally, the plating baths contain a brightening agent which is prepared by reacting an aliphatic aldehyde with an aromatic primary amine under alkaline conditions. The plating baths also contain acetaldehyde and, optionally, surface active agents.
The use of olefinic compounds as brighteners in tin and tin-lead alloy plating baths, salts of fluoboric acid or sulfuric acid, is described in U.S. Pat. No. 3,875,029. The olefinic compounds useful as brighteners are characterized by the general formula ##STR2## where R.sub.1 is carboxy, carboxamide, alkali carboxylate, ammonium carboxylate, amine carboxylate, or alkyl carboxylate, and R.sub.2, R.sub.3 and R.sub.4 are hydrogen, methyl, or lower alkyl. Such olefins are utilized in combination with a naphthalene monocarboxaldehyde.
U.S. Pat. No. 774,049 describes a process for electrolytically depositing lead peroxide on lead plates. The baths include a sulfonic acid or oxysulfonic acid derivative of methane and its hydroxy-substituted derivatives such as methyl sulfonic acid, methylene disulfonic acid, oxymethylene disulfonic acid, etc.
Tin or and tin and lead plating baths containing aromatic sulfones and mono- and poly-sulfonic acids of benzene, phenol and cresol are described in U.S. Pat. No. 2,313,371 and British Pat. No. 555,929. The aromatic sulfonic acids form soluble salts of tin and lead, and the disulfonic acids are particularly preferred because they provide extremely soluble salts.
U.S. Pat. No. 3,984,291 describes a plating bath which does not contain any fluoride or fluoborate. These plating baths are based on pyrophosphate and Rochelle salts. The bath is prepared, for example, by dissolving stannous pyrophosphate, potassium pyrophosphate and Rochelle salts (NaK tartrate) together with a lead salt such as lead tartrate or lead pyrophosphate in water. Generally, these types of electrolytic baths are operated at a pH of from 8.0 to 10.
Electroplating baths containing an alkane sulfonate ion wherein the alkyl group contains between 1 and 5 carbon atoms are described in U.S. Pat. No. 2,525,942. The sulfonate ions are the principal or sole anions in the plating bath. The metallic ions contained in the electrochemical baths may be lead, nickel, chromium, copper, zinc, iron, cobalt, cadmium and silver. The baths are described as being especially suitable for the electroplating of lead and nickel.